1. Field of the Invention
The present invention relates, generally to a method of controlling a dual clutch transmission and, more specifically, to a method for automatically controlling the gear shifting process by controlling the torque transfer of the clutches of a dual clutch transmission.
2. Description of the Related Art
Generally speaking, land vehicles require a powertrain consisting of three basic components. These components include a power plant (such as an internal combustion engine), a power transmission, and wheels. The power transmission component is typically referred to simply as the xe2x80x9ctransmission.xe2x80x9d Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. Presently, there are two typical transmissions widely available for use in conventional motor vehicles. The first, and oldest type is the manually operated transmission. These transmissions include a foot operated start-up or launch clutch to engage and disengage the driveline with the power plant and a gearshift lever to selectively change the gear ratios within the transmission. When driving a vehicle having a manual transmission, the driver must coordinate the operation of the clutch pedal, the gearshift lever and the accelerator pedal to achieve a smooth and efficient shift from one gear to the next. The structure of a manual transmission is simple and robust and provides good fuel economy by having a direct power connection from the engine to the final drive wheels of the vehicle. Additionally, since the operator is given complete control over the timing of the shifts, the operator is able to dynamically adjust the shifting process so that the vehicle can be driven most efficiently. The disadvantages of the manual transmission is that there is an interruption in the drive connection during gear shifting and that there is a great deal of required physical interaction on the part of the operator to shift gears.
The second, and newer choice for the transmission of power in a conventional motor vehicle is an automatic transmission. First and foremost, automatic transmissions offer ease of operation. The driver of a vehicle having an automatic transmission is not required to use both hands, one for the steering wheel and one for the gearshift, and both feet, one for the clutch and one for the accelerator and brake pedal in order to safely operate the vehicle. In addition, an automatic transmission provides greater convenience in stop and go situations, because the driver is not concerned about continuously shifting gears to adjust to the ever-changing speed of traffic. Although conventional automatic transmissions avoid an interruption in the drive connection during gear shifting, they suffer from the disadvantage of reduced efficiency because of the need for hydrokinetic devices, such as torque converters, interposed between the output of the engine and the input of the transmission for transferring kinetic energy therebetween.
At low speed ratios, RPM output/RPM input, torque converters multiply or increase the torque translation from the engine. During torque multiplication, the output torque is greater than the input torque for the torque converter. However, at high speed ratios there is no torque multiplication and the torque converter becomes a fluid coupling. Fluid couplings have inherent slip. Torque converter slip exists when the speed ratio is less than 1.0 (RPM input greater than than RPM output of the torque converter). The inherent slip reduces the efficiency of the torque converter.
While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in a parasitic loss, thereby decreasing the efficiency of the entire powertrain. Further, the torque converter itself requires pressurized hydraulic fluid in addition to any pressurized fluid requirements for the actuation of the gear shifting operations. This means that an automatic transmission must have a large capacity pump to provide the necessary hydraulic pressure for both converter engagement and shift changes. The power required to drive the pump and pressurize the fluid introduces additional parasitic losses of efficiency in the automatic transmission.
In an ongoing attempt to provide a vehicle transmission that has the advantages of both types of transmissions with fewer of the drawbacks, combinations of the traditional xe2x80x9cmanualxe2x80x9d and xe2x80x9cautomaticxe2x80x9d transmissions have evolved. Most recently, xe2x80x9cautomatedxe2x80x9d variants of conventional manual transmissions have been developed which shift automatically without any input from the vehicle operator. Such automated manual transmissions typically include a plurality of power-operated actuators that are controlled by a transmission controller or some type of electronic control unit (ECU) to automatically shift synchronized clutches that control the engagement of meshed gear wheels traditionally found in manual transmissions. The design variants have included either electrically or hydraulically powered actuators to affect the gear changes. However, even with the inherent improvements of these newer automated transmissions, they still have the disadvantage of a power interruption in the drive connection between the input shaft and the output shaft during sequential gear shifting. Power interrupted shifting results in a harsh shift feel which is generally considered to be unacceptable when compared to smooth shift feel associated with most conventional automatic transmissions.
To overcome this problem, other automated manual type transmissions have been developed which can be power-shifted to permit gearshifts to be made under load. Examples of such power-shifted automated manual transmissions are shown in U.S. Pat. No. 5,711,409 issued on Jan. 27, 1998 to Murata for a Twin-Clutch Type Transmission, and U.S. Pat. No. 5,966,989 issued on Apr. 4, 2000 to Reed, Jr. et al for an Electro-mechanical Automatic Transmission having Dual Input Shafts. These particular variant types of automated manual transmissions have two clutches and are generally referred to simply as dual, or twin, clutch transmissions. The dual clutch structure is most of ten coaxially and cooperatively configured so as to derive power input from a singular engine flywheel arrangement. However, some designs have a dual clutch assembly that is coaxial but with the clutches located on opposite sides of the transmissions body and having different input sources. Regardless, the layout is the equivalent of having two transmissions in one housing, namely one power transmission assembly on each of two input shafts concomitantly driving one output shaft. Each transmission can be shifted and clutched independently. In this manner, uninterrupted power upshifting and downshifting between gears, along with the high mechanical efficiency of a manual transmission is available in an automatic transmission form. Thus, significant increases in fuel economy and vehicle performance may be achieved through the effective use of certain automated manual transmissions.
The dual clutch transmission structure may include two disc clutches each with their own clutch actuator to control the engagement and disengagement of the two-clutch discs independently. While the clutch actuators may be of the electro-mechanical type, since a lubrication system within the transmission is still a necessity requiring a pump, some dual clutch transmissions utilize hydraulic shifting and clutch control. These pumps are most of ten gerotor types, and arc much smaller than those used in conventional automatic transmissions because they typically do not have to supply a torque converter. Thus, any parasitic losses are kept small. Shifts are accomplished by engaging the desired gear prior to a shift event and subsequently engaging the corresponding clutch. With two clutches and two inputs shafts, at certain times, the dual clutch transmission may be in two different gear ratios at once, but only one clutch will be engaged and transmitting power at any given moment. To shift to the next higher gear, first the desired gears on the input shaft of the non-driven clutch assembly are engaged, then the driven clutch is released and the non-driven clutch is engaged.
This requires that the dual clutch transmission be configured to have the forward gear ratios alternatingly arranged on their respective input shafts. In other words, to perform upshifts from first to second gear, the first and second gears must be on different input shafts. Therefore, the odd gears will be associated with one input shaft and the even gears will be associated with the other input shaft. In view of this convention, the input shafts are generally referred to as the odd and even shafts. Typically, the input shafts transfer the applied torque to a single counter shaft, which includes mating gears to the input shaft gears. The mating gears of the counter shaft are in constant mesh with the gears on the input shafts. The counter shaft also includes an output gear that is meshingly engaged to a gear on the output shaft. Thus, the input torque from the engine is transferred from one of the clutches to an input shaft, through a gear set to the counter shaft and from the counter shaft to the output shaft.
Gear engagement in a dual clutch transmission is similar to that in a conventional manual transmission. One of the gears in each of the gear sets is disposed on its respective shaft in such a manner so that it can freewheel about the shaft. A synchronizer is also disposed on the shaft next to the freewheeling gear so that the synchronizer can selectively engage the gear to the shaft. To automate the transmission, the mechanical selection of each of the gear sets is typically performed by some type of actuator that moves the synchronizers. A reverse gear set includes a gear on one of the input shafts, a gear on the counter shaft, and an intermediate gear mounted on a separate counter shaft meshingly disposed between the two so that reverse movement of the output shaft may be achieved.
While these power-shift dual clutch transmissions overcome several drawbacks associated with conventional transmissions and the newer automated manual transmissions, it has been found that controlling and regulating the automatically actuated dual clutch transmissions is a complicated matter and that the desired vehicle occupant comfort goals have not been achievable in the past. There are a large number of events to properly time and execute within the transmission to achieve smooth and efficient operation, not only during the power-shifting events, but also throughout the entire operating range of the transmission as well. To this point, conventional control schemes and methods have generally failed to provide this capability. Accordingly, there exists a need in the related art for better methods of controlling the operation of dual clutch transmissions.
One particular area of control improvement that is needed is in the power-shifting of the dual clutch transmission. As discussed above, power shifting is actually the automatic gear shifting process of the dual clutch transmission. The nature of the dual clutch transmission, that is, the manual style configuration discussed above that employs automatically actuated disc type clutches, requires accurate control of the clutch engagement and thus the torque transferred across them during the gear shifting process. More specifically, it is desirable to operate the clutches of the dual clutch transmission so that the automatic gear shifting process is smoothly and efficiently controlled by varying the amount of torque transferred across each clutch as the clutch driving the off-going gear is minimized and the clutch driving the on-coming clutch is maximized.
Control of the torque transferred across the clutches during the gear shifting process is required to provide smooth operation, avoid hard or noticeable lockup of the on-coming clutch to the transmission, and to provide efficient engine-to-transmission interaction during either upshifts or downshifts. The prior art dual transmission clutch control schemes are incapable of adequately providing for fine control of clutch torque transfer to satisfy this need. Current control methods do have the general capability to operate the clutches as needed. However, they lack the ability to finely control the torque transferred across the clutches to achieve the high degree of accuracy needed for smooth shifting between the gears of the transmission. Additionally, current control methods for the clutches of a dual clutch transmission generally concern themselves with simple engagement and disengagement of the clutch assemblies and fail to adequately provide for the corresponding control of all aspects of the shift process including engine speed control during the shift and the differences in upshifting and downshifting.
In that regard, some prior control methods for the gear shifting of dual clutch transmissions have attempted to overcome these inadequacies by using a control algorithm. For example, one known method provides an algorithm to control the movement of electrical clutch actuators, and thus the engagement of the clutches, to prevent torque interruption during upshifts of a dual clutch transmission. While the application of this particular algorithm is functionally adequate for its intended use, it still has certain drawbacks that leave room for improvement.
Particularly, while this and other known dual clutch transmission shifting approaches attempt to provide a power-shift in which there is no interruption of torque transfer, none of the current methods provides for a smooth and efficient torque transfer from one clutch to the other so that the shift and subsequent change in engine speed avoids a change in vehicle speed or is smooth enough to go unnoticed by the driver. For example, one existing control method applies its upshift algorithm so that, as the shift is performed, a swap of the engagement between the on-coming and off-going clutch is accomplished by increasing and decreasing the torque transferred across the respective clutches in such a manner as to follow exponential curves. This means that although no complete break or interruption in torque transfer occurs, the changeover in the clutches is non-linear so that the total torque transferred will vary as the clutches swap. The non-linear switchover of the clutches results in a total torque transfer that is uneven causing an uncontrolled change in engine speed resulting in a change in vehicle speed during the shift. This results in inefficient torque transfer and imparts poor ride characteristics to the vehicle.
Additionally, certain prior art methods utilize an engine performance map that predicts expected engine output torque and sets the clutch position based on those predictions so that this control method is reactive to predicted engine output. The drawback of this control approach is that a wide variety of variants that cannot be predicted can influence the engine torque output. These unpredicted variables can subsequently cause great inaccuracies in the control of the clutches. On the other hand, a more accurate approach would be to actively and directly control the torque transfer across clutches to effect the engine output.
Accordingly, there remains a need in the art for a method to operatively and actively control the gearshifts in a dual clutch transmission so that both upshifts and downshifts are efficiently and smoothly performed by providing control over the torque transfer of the clutches.
The disadvantages of the related art are overcome by the method of the present invention for controlling the engine speed of a vehicle having a dual clutch transmission. The method controls the torque transferred across each of the two clutches of a dual clutch transmission during a gear shift wherein the first of the two clutches is the off-going clutch and the second of the two clutches is the on-coming clutch. The method includes the steps of determining when a shift has been commanded and sensing the engine throttle position, the speed of the driven member of the off-going clutch, and the speed of the driven member of the on-coming clutch. The method then determines a target engine speed profile based on the engine throttle position, the speed of the driven member of the off-going clutch, and the speed of the driven member of the on-coming clutch. Once the target engine speed is determined, the method simultaneously controls the torque transfer across each clutch so that the torque output of the transmission will be changed over from the off-going clutch to the on-coming clutch by linearly decreasing the torque transferred across the off-going clutch while linearly increasing the torque transferred across the on-coming clutch in an inversely proportional rate to cause the engine to track the target engine speed profile. Then, the method varies the pressure applied to the on-coming clutch, once the on-coming clutch is transferring all of the output torque, to cause the engine to continue to track the target engine speed profile so that vehicle speed is maintained.
Thus, the method of the present invention controls the powershifts of the dual clutch transmission by controlling the torque transfer across the clutches in such a manner as to maintain vehicle speed during each shift event, albeit acceleration or deceleration and whether the vehicle and drivetrain are under negative or positive torque conditions. The method of the present invention considers and accounts for all the situations encountered when shifting the dual clutch transmission and thereby overcomes the inability of prior methods to control the engine and clutch speeds so that the speed and momentum of the vehicle are not interrupted. This is a substantial improvement over the prior methods of dual clutch shift control, which do not consider all the various conditions and situations in which a shift may occur. Furthermore, the shifts are also accomplished smoothly and efficiently so that there is no hard or distinctive xe2x80x9cfeelxe2x80x9d to the shift, thereby improving overall drivability and comfort of the vehicle. The smooth shifting of the dual clutch transmission is provided by the linear and inversely proportional change over from the off-going clutch to the on-coming clutch and the supplement varying of the clutch pressures to maintain the speed of the vehicle during shifting.
Other objects, features and advantages of the present invention will be readily appreciated as the same becomes better understood after reading the subsequent description taken in connection with the accompanying drawings.